1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to improving calibration of instruments and systems that use quadrature modulation.
2. Description of the Related Art
Measurement systems are oftentimes used to perform a variety of functions, including measurement of physical phenomena, measurement of certain characteristics or operating parameters of a unit under test (UUT) or device under test (DUT), testing and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical contemporary measurement system comprises a computer system, which commonly features a measurement device, or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a USB (Universal Serial Bus), a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal-conditioning devices, which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a DUT or for influencing the system being controlled. These measurement systems, which can be generally referred to as data acquisition systems (DAQs), are primarily used for converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and DAQs and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and receive digital signals to implement one or more digital I/O applications. DAQ devices may also include a Source-Measure Unit (SMU), which may apply a voltage to a DUT and measure the resulting current, or may apply a current to the DUT and measure the resulting voltage. In addition, many DAQ devices also include signal generators and/or vector signal generators for transmitting and receiving various types of signals that may be used during data acquisition and control. Some of these signals may oftentimes be transmitted/received wirelessly.
Digital wireless communications are typically implemented through the use of modulators and demodulators, which provide a necessary RF interface for systems such as cordless phones, wireless networks, and wireless peripheral devices for computers, in addition to test and control systems that may use a wireless interface to couple certain system elements. One commonly used modulation method is “quadrature modulation”, which employs two carriers out of phase by 90° and modulated by separate signals. There are also many digital encoding standards that allow for the transmission of vast amounts of data over wireless RF interfaces in shorter periods of time.
One example of quadrature modulation is quadrature amplitude modulation, where two digital bit streams may be conveyed via a digital modulation scheme by modulating the amplitudes of two carrier waves, using amplitude-shift keying (ASK). The carrier waves, which are usually sinusoid signals, are 90° out of phase with respect to each other, and are thus called quadrature carriers. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying. A finite number of at least two phases, and at least two amplitudes are used. PSK modulators are often designed using quadrature modulation principles, but are not considered quadrature amplitude modulation schemes, since the amplitude of the modulated carrier signal is constant. Another type of quadrature modulation may use frequency shift keying, where the digital information is transmitted through discrete frequency changes of the carrier waves, instead of changes in the amplitudes of the carrier signals.
Most circuits and/or signal generators that employ quadrature modulation may require calibration to correct for offset, gain imbalance and phase skew. Most available calibration methods either involve the use of a vector network analyzer with computational complications, or binary search type methods that can be lengthy.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.